Stoichiometry - Chemical Equations

Objective

This section focuses on constructing balanced chemical equations, including the correct use of state symbols and the representation of ionic equations. Understanding and writing balanced equations is fundamental to stoichiometry.

1. Balanced Chemical Equations

A balanced chemical equation accurately represents a chemical reaction by showing the reactants and products and their relative quantities. The law of conservation of mass dictates that the number of atoms of each element must be the same on both sides of the equation.

Example: The reaction of hydrogen and oxygen to form water.

Unbalanced equation: $2H_2 + O_2 \rightarrow H_2O$

Balanced equation: $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$

Key points:

• Use subscripts to indicate the number of atoms of each element in a molecule.
• Use parentheses to group reactants and products.
• Indicate the state of each substance using state symbols:
• (s) - solid
• (l) - liquid
• (g) - gas
• (aq) - aqueous (dissolved in water)

2. Ionic Equations

Ionic equations represent reactions that occur in aqueous solutions, where ions are present. These equations show the complete ionic and net ionic forms of the reaction.

Complete Ionic Equation: Shows all soluble ionic compounds dissociated into their constituent ions.

Net Ionic Equation: Shows only the species that directly participate in the reaction.

Example: The reaction between sodium chloride and silver nitrate in aqueous solution.

Complete Ionic Equation: $AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

Net Ionic Equation: $Ag^+(aq) + Cl^-(aq) \rightarrow AgCl(s)$

Steps to write a net ionic equation:

1. Write the complete ionic equation.
2. Identify the soluble ionic compounds that do not change state during the reaction.
3. Cancel out these spectator ions from the complete ionic equation.

3. Constructing Symbol Equations

To construct a symbol equation, follow these steps:

1. Write down the reactants and products.
2. Identify the elements present in the reactants and products.
3. Determine the number of atoms of each element on both sides of the equation.
4. Adjust the coefficients in front of the chemical formulas to balance the number of atoms of each element.
5. Ensure that the law of conservation of mass is satisfied.
6. Indicate the state symbols for each substance.

4. Example Problems

Problem	Balanced Equation
Combustion of methane	$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$
Reaction of hydrochloric acid with magnesium oxide	$MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$
Reaction of zinc with dilute sulfuric acid	$Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g)$

5. Practice

Practice writing balanced chemical equations for various reactions. Pay close attention to the state symbols and the correct balancing of atoms.